Method for operating a solar-thermal parabolic trough power plant

ABSTRACT

A method for operating an indirectly heated, solar-thermal steam generator and to an indirectly heated, solar-thermal steam generator are provided. A heat transfer medium is used in the solar-thermal steam generator. The supply water mass flow M is predictively controlled by a device for adjusting the supply water mass flow M. To this end, a nominal value Ms is fed to the device. A correction value K T , by which thermal storage effects of stored or withdrawn thermal energy are corrected is taken into account by the nominal value Ms.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2012/051942 filed Feb. 6, 2012 and claims benefit thereof, theentire content of which is hereby incorporated herein by reference. TheInternational Application claims priority to the German application No.10 2011 004269.5 DE filed Feb. 17, 2011, the entire contents of which ishereby incorporated herein by reference.

FIELD OF INVENTION

The invention relates to a method for operating an indirectly heatedsolar-thermal steam generator featuring a heat transfer medium, whereina desired value {dot over (M)}_(s) for the feedwater mass flow {dot over(M)} is supplied to a device for adjusting the feedwater mass flow {dotover (M)} on the flow medium side. It further relates to an indirectlyheated solar-thermal steam generator featuring a heat transfer mediumand featuring a device for adjusting the feedwater mass flow {dot over(M)}, and to a solar-thermal parabolic trough power plant comprising anindirectly heated solar-thermal steam generator.

BACKGROUND OF INVENTION

Steadily increasing energy demands and climate change must be addressedby using sustainable sources of energy. Solar energy is one suchsustainable source of energy. It does not harm the environment, isavailable in unlimited supply, and does not represent a burden on futuregenerations.

Solar-thermal power plants represent an alternative to conventionalpower generation. One power station design already known in this fieldis the so-called parabolic trough power plant. In this type of powerstation, thermo-oil is normally used as a heat transfer medium, flowingthrough the parabolic troughs and absorbing the heat supplied by thesun. The heat absorbed by the oil is normally used in an additionalsteam generator in order to generate steam. In this case, the heatedthermo-oil flows over the steam generator tubes, these being filled withsteam, and transfers its heat to the colder steam generator tubes. Thesteam which is generated in the tubes then drives a conventional steamturbine.

One possible embodiment of the steam generator is based on theonce-through principle. The feedwater entering the steam generator isessentially heated, vaporized and superheated as part of a single passin this case. The superheated medium is supplied directly to the turbinewithout any further measures (e.g. cooling via additional injection).Consequently, the live steam temperature can only be precisely adjustedby selecting the correct feedwater mass flow {dot over (M)}, andfluctuations in the feedwater quantity are directly linked tofluctuations in the live steam temperature.

The feedwater mass flow {dot over (M)} should preferably be changed atthe same time as the heat input into the steam generator via the heattransfer medium, because it is otherwise impossible reliably to avoiddeviation of the specific enthalpy of the live steam from the desiredvalue at the outlet of the steam generator. Any such unwanted deviationof the specific enthalpy makes it harder to regulate the temperature ofthe live steam emerging from the steam generator, and moreover resultsin significant material stresses and hence a reduced service life of theentire steam generator.

In solar-thermal power plant installations, inaccuracies that are causedby changes in the solar incidence, for example, must be effectivelycountered by the specification of a specifically adapted desired valuefor the feedwater mass flow, particularly in the event of a change inthe total heat absorption or in the case of load variations.

In solar-thermal energy generation systems in particular, it isgenerally impossible to assume sufficiently stable system propertiesthat can be directly attributed to a predefined constant solar energyinput. Moreover, in the context of such installations configured asindirect steam generators, solar primary output to the parabolic troughscannot be used as a free parameter to the same extent as is possible inthe case of conventionally fired boilers.

SUMMARY OF INVENTION

The object of the invention is therefore to provide a method foroperating a solar-heated once-through steam generator, beingcharacterized by a particularly high level of reliability and quality ofcontrol, particularly in the case of non-steady operations. Alsospecified is a solar-thermal steam generator which is particularlysuitable for performing the method.

The object of the invention in respect of a method is achieved by thefeatures in the claims.

In this case, the invention is based on the idea of applying a designfor predictive or advance mass flow control in the context of anindirectly heated solar-thermal steam generator in order to improve theactivation quality when adjusting the feedwater mass flow {dot over(M)}. The essence of the invention in this case is that correctionvalues which are deemed relevant should systematically be taken intoaccount when determining a suitable desired value {dot over (M)}_(s) forthe feedwater mass flow {dot over (M)}. By taking a correction valueK_(T) into account, it is possible to equalize thermal storage effectswhich occur in the form of storage or withdrawal of thermal energy inthe case of non-steady operations in particular.

This type of predictive feedwater flow control makes it possible tominimize both deviations of the specific enthalpy from the desired valueat the steam generator outlet and the resulting undesirable levels oftemperature fluctuations, in all operating conditions of the steamgenerator and particularly in transient conditions or in the context ofload variations. The necessary desired feedwater values are provided asa function of the current or soon to be expected operating condition inthis case, particularly in the context of load variations.

According to an advantageous embodiment of the method, the correctionvalue K_(T) is used to correct the thermal storage effects of thermalenergy that is stored or withdrawn relative to the tube walls of thesolar-thermal steam generator. As an alternative or in addition totaking into account the thermal energy in the tube walls, the correctionvalue K_(T) can also be advantageously used to correct the thermalstorage effects of thermal energy that is stored or withdrawn relativeto the heat transfer medium.

According to a further advantageous embodiment of the method, the totalheat quantity {dot over (Q)} of the solar-thermal steam generator isalso taken into consideration when adjusting the desired value {dot over(M)}_(s), said total heat quantity {dot over (Q)} being calculated froman enthalpy difference between the enthalpy of the heat transfer mediumat the inlet of the solar-thermal steam generator and the enthalpy ofthe heat transfer medium at the outlet of the solar-thermal steamgenerator on one hand, and the measured mass flow of the heat transfermedium at the inlet of the solar-thermal steam generator on the otherhand.

In this case, the total heat output {dot over (Q)} is advantageouslycalculated from the product of the enthalpy difference and the mass flowof the heat transfer medium. For the purpose of determining theenthalpies of the heat transfer medium at the inlet and outlet of thesteam generator, additional temperature measurements are taken at thecorresponding locations on the heat transfer medium side. In order toallow for non-steady effects of the heat conduction through the steamgenerator tube wall, these measured values for calculating theenthalpies can be slightly delayed in this case, e.g. via a PT3 element.

In order to determine the desired value of the feedwater mass flow {dotover (M)}_(s), at least for steady-state operating loads, the total heatoutput {dot over (Q)} of the heat transfer medium is divided by thewarm-up margin of the feedwater (preferred enthalpy increase). In orderto determine the warm-up margin of the feedwater, the enthalpies at thesteam generator inlet and outlet are required on the water-steam side.For this purpose, in order to determine the inlet enthalpy, thetemperature and the pressure on the flow medium side are measured at thesteam generator inlet and converted into a corresponding actualenthalpy. At the steam generator outlet, the pressure is likewisecaptured by means of measurement on the flow medium side, and thenconverted into a corresponding desired enthalpy value, taking thepreferred live steam temperature (desired temperature value) intoaccount. The warm-up margin of the feedwater is the difference betweendesired enthalpy value at the steam generator outlet and actual enthalpyat the steam generator inlet.

This makes it possible to perform a specifically adaptedprecontroller-based calculation of the required feedwater quantity onthe basis of heat flow balancing, wherein said calculation relates tothe current condition of the installation.

In a particularly advantageous development of the method, provision ismade for taking a further correction value K_(F) into account whenadjusting the desired value {dot over (M)}_(s), said correction value KFbeing used to correct storage effects of the solar-thermal steamgenerator on the water-steam side.

Furthermore, the steam generator flow which is specified by thepredictive determination of the desired feedwater value can also becorrected by means of overlaid control loops, such that the live steamdesired temperature value that is required at the steam generator outletcan actually be achieved with lasting effect. In respect of thecorrective control of the precalculated feedwater mass flow, it mustnonetheless be taken into consideration that, for reasons of controllerstability, this can only be performed very slowly and applying modestcontroller gain. Significant temporary deviations from the predetermineddesired value, which are produced as a result of physical mechanismsfollowing non-steady operation of the solar-thermal steam generatorheated by a heat transfer medium, can only be reduced slightly by thesecorrective control loops, or possibly not at all. The predictivedetermination of the desired feedwater value must therefore beconsolidated by means of additional measures, in order that thetemporary deviations from the predetermined desired value can also beminimized during rapid transient operations.

In order to achieve this objective, in addition to the correction valueK_(T), storage and withdrawal operations on the feedwater side withinthe steam generator tubes are taken into account by means of acorrection value K_(F) in this particular development of the inventivemethod. Using both correction values K_(T) and K_(F), it is possible toreact appropriately to physical mechanisms which act temporarily on theflow through the steam generator on the water-steam side duringnon-steady operation and therefore result in deviations, on the flowmedium side, of the actual enthalpy at the outlet of the steam generatorfrom the predefined desired value. In order to determine the correctionvalue K_(T), it is necessary to distinguish between two differentmechanisms.

During non-steady operations, values relating to thermodynamicconditions such as e.g. the live steam temperature, the pressure (andtherefore the boiling temperature of the flow medium in subcriticalcases) and the feedwater temperature generally change on the flow mediumside in the steam generator. As a result of these changes, the materialtemperature of the steam generator tubes likewise is not constant, andgoes up or down according to direction. Consequently, thermal energy isstored in or withdrawn from the tube walls. Compared with the balancedtotal heat output {dot over (Q)} which the heat transfer medium releasesto the feedwater, more or less heat is therefore temporarily availablefor the steam generation process depending on the direction of materialtemperature change. In the case of a predefined desired enthalpy valueand/or live steam desired temperature value at the outlet of thesolar-thermal steam generator, it is therefore essential for this notinconsiderable influence to be taken into account in the control systemfor the purpose of advance calculation of the necessary desiredfeedwater mass flow value {dot over (M)}_(s).

This physical effect can be reproduced in terms of control engineeringby means of a first-order differentiating circuit (DT1 element). Anaverage material temperature of all steam generator tubes must bedefined and used as an input signal of the differentiating circuit. Forexample, the average material temperature here can be determined fromthe known process variables of live steam temperature, system pressureand feedwater temperature, possibly also taking the measuredtemperatures of the heat transfer medium into consideration. If thisaverage material temperature now changes, and if the output of thedifferentiating circuit is multiplied by the mass of the total steamgenerator tubes and the specific heat capacity of the evaporatormaterial and the reciprocal value of the time constant of thedifferentiating circuit, the quantities of heat that are stored in orwithdrawn from the tube wall can be quantified. By selecting a suitabletime constant for this differentiating circuit, the temporal behavior ofthe described storage effects can be simulated with relative accuracy,such that this additional effect, caused by non-steady operation, ofheat of the metal masses being stored or withdrawn can be calculateddirectly. This is equally applicable to subcritical and supercriticalsystems.

Alternatively, it is also conceivable to measure the materialtemperature directly at characteristic locations of the steam generatortubes. In these circumstances, a change in the metal temperature can betaken into account directly. In this case, both the number ofdifferentiating circuits and their corresponding amplification factors(essentially the mass of the steam generator tubes) would have to beadapted to the number of metal temperature measurements. Although thisvariant involves greater cost in terms of measuring technology, it hasthe advantage that the quantities of heat being stored or withdrawn aredetermined more accurately as a result.

In addition to these storage or withdrawal operations in respect ofadditional heat energy from the material of the steam generator tubes,storage and withdrawal operations in respect of thermal energy of theheat transfer medium are likewise a factor that must not be ignoredduring non-steady operation of the solar-thermal steam generator. If theaverage temperature level of the heat transfer medium decreases in thewhole steam generator, for example, further heat is released and isadditionally absorbed by the steam generator tubes. Compared with thebalanced total heat output {dot over (Q)}, more heat output is thereforeavailable for the steam generating process. It follows that less heat isavailable in the opposite case. These effects must also be taken intoaccount if the necessary desired feedwater mass flow value {dot over(M)}_(s) is to be predetermined correctly. Here likewise, the additionalheat output that is stored or withdrawn can preferably be determined bymeans of a first-order differentiating circuit (DT1 element) using asuitable time constant. To this end, provision is likewise made fordetermining an average temperature of the heat transfer medium, for useas an input signal of this DT1 element. The average temperature canpreferably be functionally determined from the measured temperatures ofthe heat transfer medium at the steam generator inlet and outlet. Forthe purpose of determining this average temperature of the heat transfermedium, it is also conceivable to use further known process parameters,other characteristic values or even additional temperature measurementsalong the flow path of the heat transfer medium in the steam generator.The output signal of the DT1 element is preferably multiplied by thevolume that is occupied by the heat transfer medium in the controlvolume of the steam generator, the density of the heat transfer mediumand the specific heat capacity of the heat transfer medium, and by thereciprocal value of the time constant of the DT1 element. It should benoted in this case that both the density and the specific heat capacityof the heat transfer medium are temperature-dependent, and can possiblybe determined using the average temperature that has already beencalculated for the heat transfer medium, for example. In thesecircumstances, the additional heat output is also quantified in thiscase.

Both of the additional heat quantities from tube wall and heat transfermedium are then added to give the correction value K_(T), which must besubtracted from the balanced total heat output {dot over (Q)} in orderto determine the desired feedwater mass flow value {dot over (M)}_(s).

The second correction value K_(F), which correctively acts directly onthe desired value of the feedwater mass flow {dot over (M)}_(s), alsocompensates effectively for further disturbing influences, which areproduced due to non-steady operation, in the water-steam circuit of thesolar-thermal steam generator. In this context, disturbances in thefeedwater temperature at the inlet of the solar-thermal steam generatorhave a significant effect on its flow. Specifically, this means that adecrease in the feedwater temperature is associated with a reduction inthe specific volume of the flow medium in the inlet region of thesolar-thermal steam generator. This results in a requirement foradditional feedwater, which must fill the now unused volume of the steamgenerator tubes (feedwater is brought in). If the feedwater temperatureincreases, however, the reverse mechanism is triggered. If the feedwatertemperature at the inlet of the solar-thermal steam generator nowundergoes changes due to non-steady operation, the resulting storage andwithdrawal operations on the feedwater side mean that the inlet andoutlet mass flows of the solar-thermal steam generator are notidentical. This has a direct influence on the steam generator outletenthalpy (live steam temperature) which, under these circumstances, maynot remain constant even if the heat input remains the same. Thereforethe effects of fluctuating feedwater temperatures at the inlet of thesolar-thermal steam generator must likewise be balanced out bycountermeasures in the form of determining the desired feedwater value(increasing or decreasing the desired feedwater mass flow value {dotover (M)}_(s)). If an additional first-order differentiating circuit isalso used here, it is however possible effectively to reduce theenthalpy fluctuations (fluctuations of the live steam temperature) atthe outlet of the solar-thermal steam generator by selecting a suitableinput signal (e.g. inlet overcooling of the steam generator, inletenthalpy of the steam generator or the feedwater temperature itself), anappropriate time constant and a suitable amplification.

In a preferred embodiment of the invention, the solar-thermal steamgenerator is integrated into a solar-thermal parabolic trough powerplant featuring a number of parabolic troughs, by means of which theheat transfer medium is subjected to solar-thermal heating. If theinventive determination of the desired feedwater value is used inoil-heated solar-thermal steam generators, constant live steamtemperatures can even be ensured in distinctly non-steady operatingconditions, such as increasingly occur in solar-heated power plants(e.g. cloud passage). In addition to a consequently reliable means ofcontrol during changeable weather conditions, the availability of theentire power plant installation can be improved by means of amaterial-preserving design. Moreover, the design according to theinvention is also suitable for modular use in a plurality ofsolar-heated steam generators in a single parabolic trough power plant.The design can also be used without significant changes in combinationwith other components such as injection coolers, for example.

The heat transfer medium can be a thermo-oil or a salt melt, forexample. The use of suitable metal melts as a heat transfer medium isalso possible.

The object of the invention in respect of a device is achieved by thefeatures in the claims. It is particularly advantageous in this case ifthe indirectly heated solar-thermal steam generator is integrated into asolar-thermal parabolic trough power plant and connected to a number ofparabolic troughs for the supply of the superheated heat transfermedium, wherein said parabolic troughs can be directly subjected tofocused solar incidence. The object of the invention in respect of adevice is also achieved by the features in the claims.

In particular, the advantages of the invention consist in being able tocorrect the desired value determined in the context of predictive massflow control for the feedwater mass flow {dot over (M)}, by taking intoaccount the temporal dissipation of the enthalpy, temperature or thedensity of the feedwater at the input of the steam generator, whereindue consideration can also be given to storage and withdrawal operationsin respect of thermal energy of the tube material. This makes itpossible to determine a specifically adapted desired value for thefeedwater mass flow {dot over (M)} with a high level of precision,particularly in the event of load variations or other transientoperations.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are explained in greater detailin FIGS. 1,2,3 and 4, in which:

FIG. 1 shows a schematic illustration of an indirectly heatedsolar-thermal steam generator featuring feedwater flow control forsteady-state operation,

FIG. 2 shows a schematic illustration of an indirectly heatedsolar-thermal steam generator in a development for non-steady operation,featuring predictive determination of a desired value for feedwater massflow,

FIG. 3 shows a schematic illustration of an indirectly heatedsolar-thermal steam generator in an alternative embodiment fornon-steady operation, featuring predictive determination of a desiredvalue for feedwater mass flow,

FIG. 4 shows a schematic illustration of an indirectly heatedsolar-thermal steam generator in a particular development for non-steadyoperation, featuring predictive determination of a desired value forfeedwater mass flow.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a schematic diagram of a control circuit for determining adesired feedwater value for steady-state operation of a solar-thermalsteam generator 3 in a parabolic trough power plant 1. The indirectlyheated solar-thermal steam generator 3, a device for adjusting thefeedwater mass flow 5, and a feedwater flow control 11 are illustratedas important components. The circuit is part of a solar-thermalparabolic trough power plant 1, which also features parabolic troughsand a power plant section comprising a steam turbine (not shown).

The solar-thermal steam generator 3 is configured for the supply ofthermo-oil 4 as a heat transfer medium, and is connected on thethermo-oil side to an incoming thermo-oil supply line 13 and an outgoingthermo-oil outflow line 14. The solar-thermal steam generator 3 is alsoconnected on the feedwater side to an incoming feedwater supply line 15and to an outgoing steam line 16. Heat energy can therefore betransferred from the highly heated thermo-oil 4 to the feedwater bymeans of the solar-thermal steam generator 3, thereby vaporizing thefeedwater.

The solar-thermal steam generator 3 is also configured to allow acontrolled delivery of feedwater. The device for adjusting the feedwatermass flow 5 comprises a servomotor 18, which activates a throttle valve19 such that the feedwater quantity or the feedwater mass flow {dot over(M)} that is transported from a feedwater pump 17 in the direction ofthe solar-thermal steam generator 3 can be adjusted by activating thethrottle valve 19 correspondingly. For the purpose of determining acurrent characteristic value for the supplied feedwater mass flow {dotover (M)}, a measuring device 20 for determining the feedwater mass flow{dot over (M)} through the feedwater line is connected upstream of thethrottle valve 19 in the feedwater supply line. The servomotor 18 isactivated via a control element 21 which, on the input side, receives adesired value {dot over (M)}_(s) for the feedwater mass flow {dot over(M)}, said value being supplied via a data line 22, and a current actualvalue of the feedwater mass flow {dot over (M)} as determined by themeasuring device 20. A difference between these two signals indicates acorrection requirement, such that a corresponding correction of thethrottle valve 19 is effected via the activation of the motor 18 if theactual value deviates from the desired value.

For the purpose of determining a desired value {dot over (M)}_(s) forthe feedwater mass flow {dot over (M)}, the input side of the data line22 is connected to the feedwater flow control 11, which is so configuredas to specify the desired value {dot over (M)}_(s) for the feedwatermass flow {dot over (M)}.

The desired value {dot over (M)}_(s) is determined with reference to aheat flow balance in the solar-thermal steam generator 3, and is basedon the relationship between the heat flow that is currently beingtransferred by the thermo-oil 4 into the feedwater in the solar-thermalsteam generator 3 on one hand, and a predefined desired value for anenthalpy increase of the feedwater with regard to the preferred livesteam condition on the other. The feedwater flow control 11 features adividing element 23 for the purpose of supplying the desired value {dotover (M)}_(s).

The numerator is supplied to the dividing element 23 by a functionmodule 24. The function module 24 calculates the heat output {dot over(Q)} that has been introduced into the solar-thermal steam generator 3,by multiplying the enthalpy difference of the thermo-oil 4 at the inletand at the outlet of the solar-thermal steam generator 3 by the massflow of the thermo-oil 4. In order to provide the mass flow of thethermo-oil 4 before it enters the solar-thermal steam generator, thefunction module 24 is connected to a measuring device 31, which isconnected into the thermo-oil supply line 13. In order to provide theenthalpy difference of the thermo-oil 4, the function module 24 isconnected to an analysis unit 25.

Connected to the analysis unit 25 are a first module 27 for calculatingthe enthalpy at the inlet of the solar-thermal steam generator 3 and asecond module 28 for calculating the enthalpy at the outlet of thesolar-thermal steam generator 3. The first module 27 is connected to afirst oil temperature measuring unit 29, which is connected into thethermo-oil supply line 13 at the inlet of the solar-thermal steamgenerator 3, and the second module 28 is connected to a second oiltemperature measuring unit 30, which is connected into the thermo-oiloutflow line 14 at the outlet of the solar-thermal steam generator 3. Inthis case, the inlet of the thermo-oil designates a region of thesolar-thermal steam generator 3 in which heat energy of the thermo-oilis not yet transferred to the feedwater. Similarly, the outlet of thethermo-oil is a region of the solar-thermal steam generator 3 in whichthe heat energy of the thermo-oil is transferred to the feedwatercorrespondingly. The measurement of the mass flow of the thermo-oiltakes place before it enters the generator.

For the purpose of supplying the denominator, i.e. the characteristicvalue for the preferred enthalpy increase (warm-up margin), the inputside of the dividing element 23 is connected to a function module 32.The function module 32 generates the enthalpy difference from acalculated desired enthalpy at the outlet of the solar-thermal steamgenerator 3 and a measured actual enthalpy of the feedwater before itenters the solar-thermal steam generator 3.

The actual value of the current enthalpy of the feedwater before itenters the solar-thermal steam generator 3 is determined by an analysisunit 33 and transferred to the function module 32. For the purpose ofdetermining measured data, the analysis unit 33 is also connected to apressure measuring device 35 and a temperature measuring device 36, bothof which are connected into the feedwater supply line 15.

The desired enthalpy at the outlet of the solar-thermal steam generator3 is calculated by a function module 34. This desired value isdetermined from the preferred live steam temperature (live steam desiredtemperature value) and the measured pressure at the outlet of thesolar-thermal steam generator 3. The data relating to the pressure atthe outlet of the solar-thermal steam generator 3 is supplied to thefunction module 34 by means of a pressure sensor 37.

The enthalpy increase of the flow medium, which increase is required inthe solar-thermal steam generator 3 as a function of the preferred livesteam condition, is therefore determined by subtraction in the functionmodule 32 and used as a denominator in the dividing element 23. Thedividing element 23 calculates the required mass flow signal therefrom.

In an extension of FIG. 1, FIG. 2 shows a control circuit diagram forpredictive determination of the desired feedwater value for non-steadyoperation.

During non-steady operation, values relating to thermodynamic statessuch as e.g. the live steam temperature, the pressure (and therefore theboiling temperature of the flow medium in subcritical systems) and thefeedwater temperature generally change in the steam generator. As aresult of these changes, the material temperature of the steam generatortubes likewise is not constant, and goes up or down according todirection. Consequently, thermal energy is stored in or withdrawn fromthe tube walls. Compared with the balanced heat of the thermo-oil, moreor less heat is therefore temporarily available for the steam generationprocess of the flow medium depending on the direction of materialtemperature change.

In the case of a predefined desired enthalpy value and/or live steamdesired temperature value at the outlet of the solar-thermal steamgenerator 3, it is therefore essential for this not inconsiderableinfluence to be taken into account in the control system for the purposeof advance calculation of the required feedwater mass flow. According tothe invention, this is effected by means of a correction value K_(T).The correction value K_(T) is a characteristic heat flow value, by meansof which the storage and withdrawal effects of the steam generator tubescan be determined. This is equally applicable to subcritical andsupercritical systems.

For the purpose of taking the correction value K_(T) into account, in anextension of FIG. 1, a subtractor 40 is provided in FIG. 2, beingconnected between the function module 24 and the dividing element 23.The subtractor 40 determines the difference between the heat output {dotover (Q)} that has been introduced into the steam generator (total heatabsorption) and the correction value K_(T), and forwards the result tothe dividing element 23 as a corrected heat quantity that has beenintroduced {dot over (Q)}_(Korr).

The correction value K_(T) is supplied to the subtractor by adifferentiating circuit 41. An average material temperature of all steamgenerator tubes must be defined and used as an input signal for thedifferentiating circuit 41. For example, the average materialtemperature here can be determined from the known process variables oflive steam temperature, system pressure and feedwater temperature. Ifthis average material temperature now changes, and if this temporalchange (evaluated by the differentiating circuit 41) is multiplied bythe mass of the total steam generator tubes and the specific heatcapacity of the evaporator material, the quantities of heat that arestored in or withdrawn from the tube wall can be quantified in the formof the correction value K_(T). By selecting a suitable time constant ofthe differentiating circuit 41, the temporal behavior of the describedstorage effects can be simulated with relative accuracy, such that thisadditional effect, caused by non-steady operation, of heat of the metalmasses being stored or withdrawn can be calculated directly.

FIG. 3 shows an alternative embodiment to that shown in FIG. 2 of thepredictive determination of a desired value for the feedwater mass flow.In addition to the differentiating circuit 41, which determines thestorage effects in metal masses, a further differentiating circuit 45 isprovided for the purpose of determining the storage effects of thermalenergy in relation to the thermo-oil. To this end, the differentiatingcircuit 45 likewise analyzes known process parameters relative to time,e.g. the measured oil temperature at the steam generator inlet or at thesteam generator outlet, or an average oil temperature that has beenfunctionally derived from these two measured temperature values. If theoutput signal of this differentiating circuit 45 is now multiplied bythe total oil volume, the density and the specific heat capacity of theoil, it is likewise possible to quantify the heat quantities that arestored or withdrawn relative to the thermo-oil. Using a suitable timeconstant of the differentiating circuit 45, the temporal behavior of thestorage effects in relation to the thermo-oil can be simulated withrelative accuracy. This additional effect, caused by non-steadyoperation, of heat being stored or withdrawn relative to the thermo-oilcan therefore be calculated directly.

The output signals of the differentiating circuit 41 and thedifferentiating circuit 45 are summed in a function element 45 to formthe correction value K_(T), which is supplied as an input signal to thesubtractor 40.

FIG. 4 shows a schematic illustration of an indirectly heatedsolar-thermal steam generator in a particular development for non-steadyoperation, featuring predictive determination of a desired value for thefeedwater mass flow.

Disturbances in the feedwater temperature at the inlet of thesolar-thermal steam generator 3 have a significant effect on its flow.Specifically, this means that a decrease in the feedwater temperature isassociated with a reduction in the specific volume of the flow medium inthe inlet region of the solar-thermal steam generator 3. This results ina requirement for additional feedwater, which must fill the now unusedvolume of the steam generator tubes (feedwater is brought in). If thefeedwater temperature increases, however, the reverse mechanism istriggered.

If the feedwater temperature at the outlet of the solar-thermal steamgenerator now undergoes changes due to non-steady operation, theresulting storage and withdrawal operations on the feedwater side meanthat the inlet and outlet mass flows of the solar-thermal steamgenerator 3 are not identical. This has a direct influence on the steamgenerator outlet enthalpy (live steam temperature) which, under thesecircumstances, may not remain constant even if the heat input remainsthe same. Therefore the effects of fluctuating feedwater temperatures atthe inlet of the solar-thermal steam generator 3 are likewise balancedout by countermeasures in the form of determining the desired feedwatervalue (increasing or decreasing the feedwater mass flow). This iseffected by means of the correction value K_(F).

In an extension of FIG. 3, FIG. 4 further shows an adder 42, which isconnected into the data line 22 and corrects the desired value {dot over(M)}_(s) by the correction value K_(F). The adder is a component of thefeedwater flow control 11. The correction value K_(F) is supplied to theadder 42 via a differentiating circuit 43. The input signal of thedifferentiating circuit 43 in this case comprises data such as e.g.inlet overcooling of the steam generator, inlet enthalpy of the steamgenerator or the feedwater temperature itself. As a function of thisinput signal, the differentiating circuit 43 is parameterized using anappropriate time constant and a suitable amplification, in ordereffectively to reduce the enthalpy fluctuations (fluctuations of thelive steam temperature) at the outlet of the solar-thermal steamgenerator 3.

If the inventive determination of the desired feedwater value is appliedin oil-heated solar-thermal steam generators 3, constant live steamtemperatures in BENSON mode can even be ensured in distinctly non-steadyoperating conditions, such as increasingly occur in solar-heated powerplants (e.g. cloud passage). In addition to a consequently reliablemeans of control during changeable weather conditions, the availabilityof the entire installation can be improved by means of amaterial-preserving design. Moreover, the design according to theinvention is also suitable for modular use in a plurality ofsolar-heated steam generators in a single parabolic trough power plant.The design can also be used without significant changes in combinationwith other components such as injection coolers, for example.

By virtue of the method according to the invention, precisely therequired feedwater mass flow through the solar-thermal steam generator 3is supplied at all times, as a function of the available heat outputfrom solar radiation, in order to ensure the required and/or preferredlive steam mass flow at the outlet of the solar-thermal steam generator3 (live steam temperature), even during non-steady operation and inparticular in the event of cloud passage through the solar field.

1-12. (canceled)
 13. A method for operating an indirectly heatedsolar-thermal steam generator featuring a heat transfer medium,comprising: supplying a desired value {dot over (M)}_(s) for thefeedwater mass flow {dot over (M)} to a device for adjusting thefeedwater mass flow {dot over (M)}, wherein a first correction valueK_(T) is taken into account when setting the desired value {dot over(M)}_(s) for the feedwater mass flow {dot over (M)}, whereby thermalstorage effects of thermal energy that is stored or withdrawn relativeto the steam generator is corrected.
 14. The method as claimed in claim13, wherein the thermal storage effects of thermal energy that is storedor withdrawn relative to a plurality of tube walls of the solar-thermalsteam generator are corrected by means of the first correction valueK_(T).
 15. The method as claimed in claim 13, wherein the thermalstorage effects of thermal energy that is stored or withdrawn relativeto the heat transfer medium are corrected by means of the firstcorrection value K_(T).
 16. The method as claimed in claim 13, wherein atotal heat quantity {dot over (Q)} of the solar-thermal steam generatoris taken into account when setting the desired value {dot over (M)}_(s),and is calculated from the product of an enthalpy difference between afirst enthalpy of the heat transfer medium at an inlet of thesolar-thermal steam generator and a second enthalpy of the heat transfermedium at an outlet of the solar-thermal steam generator, and a measuredmass flow of the heat transfer medium ahead of the inlet of thesolar-thermal steam generator.
 17. The method as claimed in claim 13,wherein a second correction value K_(F) is also taken into account whensetting the desired value {dot over (M)}_(s), and wherein the secondcorrection value K_(F) is used to correct feedwater quantities that arestored or withdrawn relative to a plurality of steam generator tubes ofthe solar-thermal steam generator.
 18. The method as claimed in claim17, wherein the second correction value K_(F) is determined using afeedwater inlet overcooling.
 19. The method as claimed in claim 17,wherein the second correction value K_(F) is determined using afeedwater inlet enthalpy.
 20. The method as claimed in claim 17, whereinthe second correction value K_(F) is determined using a feedwatertemperature.
 21. The method as claimed in claim 13, wherein thesolar-thermal steam generator is integrated into a solar-thermalparabolic trough power plant featuring a plurality of parabolic troughs,by means of which the heat transfer medium is subjected to solar-thermalheating.
 22. The method as claimed in claim 13, wherein the heattransfer medium is selected from the group consisting of thermo-oil, asalt melt and a metal melt.
 23. An indirectly heated solar-thermal steamgenerator, comprising: a heat transfer medium; and a device foradjusting the feedwater mass flow {dot over (M)}, wherein the device isgoverned with reference to a desired value {dot over (M)}_(s) for thefeedwater mass flow {dot over (M)}, and wherein an associated feedwaterflow control is so configured as to specify the desired value {dot over(M)}_(s) by means of the method as claimed in claim
 13. 24. Theindirectly heated solar-thermal steam generator as claimed in claim 23,wherein the solar-thermal steam generator is integrated into asolar-thermal parabolic trough power plant and is connected to aplurality of parabolic troughs for the supply of the superheated heattransfer medium, and wherein the plurality of parabolic troughs aredirectly subjected to focused solar incidence.
 25. The indirectly heatedsolar-thermal steam generator as claimed in claim 23, wherein the heattransfer medium is selected from the group consisting of thermo-oil, asalt melt and a metal melt.
 26. A solar-thermal parabolic trough powerplant, comprising: an indirectly heated solar-thermal steam generator asclaimed in claim 23.